A variety of approaches have been developed to permit controlled, sustained release of a biologically active agent into a subject. Examples of controlled release systems include the polymeric compositions described in U.S. Pat. Nos. 4,938,763; 5,278,201 and 5,278,202. The compositions described in these patents are administered to the body of a subject in a flowable state. Once in the body, the composition coagulates or cures to form a solid implant.
One polymeric composition includes a thermoplastic polymer or copolymer, an organic solvent and a biologically active agent. The thermoplastic polymer is biocompatible, biodegradable and substantially insoluble in aqueous body or tissue fluids. The organic solvent is also biocompatible and miscible to dispersible in aqueous body or tissue fluids. The polymeric composition is flowable and can be introduced into the body using a syringe, for example. When the polymeric composition comes into contact with an aqueous medium, such as body or tissue fluid, the solvent dissipates or diffuses into the aqueous medium. Concurrently, the substantially insoluble thermoplastic polymer precipitates or coagulates to form a solid implant. As the thermoplastic polymer precipitates or coagulates to form the solid matrix, the active agent is trapped or encapsulated throughout the polymeric matrix. The biologically active agent is then released by dissolution or diffusion through the polymeric matrix and/or the biologically active agent is released as the matrix biodegrades.
However, the formation of the solid matrix from the flowable delivery system is not instantaneous. Typically the process can occur over a period of minutes to several hours. During this period, the rate of diffusion of the biologically active agent from the coagulating polymeric composition may be much more rapid than the rate of release that occurs from the subsequently formed solid matrix. This initial "burst" of biologically active agent that is released during implant formation may result in the loss or release of a large amount of the active agent. If the active agent is particularly toxic, this initial release or burst is likely to lead to toxic side effects and may damage adjacent tissues.
Therefore, a flowable delivery system that allows for the in situ formation of an implant while reducing or eliminating the initial "burst effect" would represent a significant advancement. Such delivery systems would permit higher concentrations of an active agent to be safely incorporated into an implant. The efficacy of such systems would also be improved, since a much greater percentage of the active agent would remain in the implant for sustained release and not be lost during the initial burst.